A technology of applying an electric double layer capacitor, in which quick charge is possible and a charge-and-discharge cycle life is long, as an electricity storage device for an electric motor coach, such as a hybrid vehicle, has attracted attention.
In order to constitute an electricity storage device having a required capacity, each capacitor module is formed by connecting a plurality of capacitor cells in series and a plurality of the capacitor modules are connected with each other in a row or in series.
In JP06-343225A which is a Japanese Patent Laid-Open Publication, an electricity storage device in which in order to adjust assigned voltages of such capacitors, a terminal voltage of each capacitor is compared with a stipulated voltage previously set and when the terminal voltage reaches the stipulated voltage, a bypass circuit for bypassing the capacitor is closed so as to avoid a further charge is disclosed, and it is well known.
In this case, however, when the speed of a vehicle is reduced or the like, energy is regenerated, more specifically, a rotary electric machine is caused to generate electricity as an electric generator and the capacitor is charged with the electric power generated. Then, if the electric power regenerated is little, there will sometimes be the case that a difference of the voltages among the capacitors is not solved indefinitely.
In this case, it is required to periodically carry out charging processes in which the assigned voltages of a capacitor cell are equalized (initialized) with a limit value. Also, external charging equipment is required and charging time is required before the vehicle is driven. This is a disadvantage which can be anticipated.
The present invention is directed to solve such problems.
More specifically, an advantage of the present invention is to restrain a difference of assigned voltages among respective capacitors by controlling charge and discharge of the capacitors based on an average value of the voltages among the capacitors.